Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 31, 2018
Valid: August 1-5 (Wednesday-Sunday)

Summary

An upper level pattern similar to last week’s will keep a Slight risk of an ozone exceedance through Friday, before the westward building of the Bermuda high will gradually increase ozone conducive conditions in western portions of the region, increasing the risk to Marginal over the weekend. Widespread, and sometimes heavy precipitation will continue to impact the Mid-Atlantic throughout the rest of the work week. The presence of the Bermuda high centered in the western Atlantic Ocean and a stalled upper level trough over the Mississippi River Valley will allow a conveyor belt of moisture to surge northward into the Mid-Atlantic, resulting in a stretch of unsettled days. Strong southerly flow aloft, cloudy skies, and widespread precipitation will keep ozone formation minimal across the entire region through Friday. Saturday and Sunday will be transition days as the synoptic pattern begins to shift towards an ozone friendly pattern, rising the risk of an exceedance to Marginal over the weekend, with the focus on the western part of the Mid-Atlantic. The trend is toward a classic Bermuda High ozone-conducive pattern, but the greatest risk looks to be mid-next week, beyond the medium range period. Over the weekend, weak upper level ridging will keep a surface trough along the vicinity of the I-95 Corridor and lingering shortwave energy aloft, which will limit ozone formation in the eastern part of the region. A mix of sun and clouds, light/calm surface winds, near average temperatures, and flow aloft weakening as it shifts southwesterly/westerly will be favorable for ozone formation in western PA. The primary forecast questions will be how quickly the air mass will modify under mid-level ridging, the coverage of precipitation and cloud cover, and air mass characteristics across the Ohio River Valley (source region for PIT back trajectories) and western PA. The risk of an exceedance will rise to Marginal, with a focus on western PA. Conditions favorable for ozone formation will continue to develop on Sunday across the western Mid-Atlantic as the mid-level ridge continues to build westward. A shift to westerly flow aloft, mostly sunny skies, and light surface winds will be favorable for ozone formation in western parts of the region. The development and coverage of afternoon precipitation will be the primary forecast questions on Sunday. The risk of an ozone exceedance will remain Marginal due to the uncertainty in the precipitation forecast and air mass characteristics, with a focus on the western parts of the NMA and CMA.

NWP Model Discussion

Although the NAM continues to be a slight outlier compared to the GFS and ECMWF, the weather models are in close agreement on the main synoptic features throughout the medium range period. The upper level trough currently over the Mississippi River Valley will remain in place and gradually weaken through the end of the work week, as it is sandwiched between two ridges of high pressure: the first over the Four Corners region to the west and the other a Bermuda high over the western Atlantic. By 12Z Wednesday, embedded shortwave energy will be lifting northeastward, out of the base of the trough, across the Ohio River Valley and into the Great Lakes region. Weak shortwave energy lingering throughout the base of the trough will keep this feature in place, albeit weakened, through 00Z Friday. On Friday, the westward building of the Bermuda high and eastward building of the four corners ridge will begin to pinch off the longwave trough, gradually filling it in by 18Z Saturday. The weakening of the Mississippi River Valley trough will allow the Bermuda high to build westward over the eastern U.S. at mid-levels as early as Friday. Beginning on Saturday and continuing on Sunday, the center of the ridge will push into the Southeastern U.S. By 00Z Monday, the majority of the eastern CONUS will be under the Bermuda high. The upper level ridge will be relatively weak, however, with a few shortwave perturbations remaining across the Mid-Atlantic over the weekend, keeping the potential for unsettled conditions across parts of the region.

The Dailies

Day 1 (Wednesday): Widespread unsettled conditions will impact the entire Mid-Atlantic on Wednesday, as the region will be in the warm sector of a weak low pressure system lifting northeastward through the Ohio River Valley and Great Lakes region. Strong southerly flow will pull a tropical air mass (precipitable water values near or greater than 2 inches) into the Mid-Atlantic, promoting widespread, locally heavy precipitation and mostly cloudy skies. The WPC has placed the entire region under Marginal risk of excessive rainfall with a strip of Slight risk of excessive rainfall from the Gulf Coast stretching northeastward along the spine of the Appalachians into northeastern PA. The NC and BAMS-CMAQ air quality models are in agreement that Good ozone will blanket the region under unsettled conditions. The BAMS-MAQSIP appears to be unreasonably aggressive as it develops scattered Moderate with pockets of mid-to-upper Moderate ozone throughout PA. Given the flow pattern and widespread nature of unsettled conditions, ozone levels reaching the Moderate range seems extremely unlikely in the Mid-Atlantic. This risk of an ozone exceedance will remain Slight on Wednesday.

Day 2 (Thursday): Widespread, locally heavy precipitation will continue on Thursday as the weak cold front associated with the low pressure system edges into the Mid-Atlantic. The WPC again has the entire Mid-Atlantic under Marginal risk for excessive rainfall with a strip of Slight risk of excessive rainfall from the Gulf Coast stretching northeastward along the spine of the Appalachians into northeastern PA. Similar to Wednesday, widespread precipitation, strong southerly flow, and mostly cloudy skies will keep ozone formation minimal across the region. The air quality models are mostly in agreement as they develop Good air quality throughout the entire Mid-Atlantic. The NC-NAM models are the most aggressive on Thursday as they develop scattered upper Good/Moderate ozone behind the slow moving cold front, across western PA. Another day of widespread unsettled conditions and strong southerly flow will keep a Slight risk of an ozone exceedance on Thursday.

Day 3 (Friday): Widespread unsettled conditions will persist on Friday as Thursday’s weak cold front stalls and slowly dissipates across the Mid-Atlantic. The Bermuda high will begin to build westward at mid-levels, keeping southerly flow west of the dissipating frontal boundary. This flow pattern will continue to act as a conveyor belt, bringing an abundance of tropical moisture northward into the Mid-Atlantic. Yet another day of widespread precipitation and mostly cloudy skies will prevent any significant ozone formation from taking place on Friday. The BAMS air quality models are in agreement with Good ozone blanketing most of the Mid-Atlantic with a strip of Moderate ozone creeping into western PA. This long stretch of unsettled conditions and strong southerly flow makes it hard to believe that ozone could reach into the Moderate range anywhere in the Mid-Atlantic. As a result, the risk of an ozone exceedance will remain Slight.

Day 4-5 (Saturday-Sunday): Saturday and Sunday will be transition days as the synoptic pattern begins to shift. The Bermuda high over the western Atlantic will continue to build westward over the eastern U.S., but a surface trough along the eastern Mid-Atlantic and lingering shortwave energy across the region (under weak upper level ridging) will keep unsettled conditions throughout the weekend. On Saturday, the majority of precipitation will be concentrated to the CMA with more scattered showers throughout the rest of the region. A mix of sun and clouds, light/calm surface winds, near average temperatures, and flow aloft weakening as it shifts southwesterly/westerly will be favorable for ozone formation across interior portions of the NMA. In the eastern portions, persistent strong southerly onshore flow should continue to limit ozone formation. Across the SMA, continued onshore flow, scattered showers, and mostly cloudy skies will also keep ozone formation minimal. The BAMS air quality models respond to these conditions with widespread Good ozone throughout the entire CMA and SMA. The BAMS models also respond to ozone conducive conditions throughout the interior NMA as they develop Moderate ozone throughout the western NMA and CMA (roughly west of I-81), with isolated USG ozone in the PIT metro area. The primary forecast questions on Saturday will be how quickly the air mass will modify under mid-level ridging, the coverage of precipitation and cloud cover across the NMA, and air mass characteristics across the Ohio River Valley (source region for PIT back trajectories) and western PA. As a result, the risk of an ozone exceedance will increase to Marginal with a focus on western PA.

Conditions favorable for ozone formation will continue to develop slowly on Sunday as the mid-level ridge continues to build westward into the CONUS. A shift to westerly flow aloft, mostly sunny skies, and light surface winds will be favorable for ozone formation again in the interior NMA and CMA. A surface trough across the eastern half of the NMA and CMA will allow the opportunity for afternoon precipitation, however, limiting ozone formation, along with another day of fast southerly onshore flow aloft. The development and coverage of afternoon precipitation across the western portions of the NMA and CMA will be the primary forecast questions on Sunday. Across the SMA, continued onshore flow and a mix of sun and clouds will continue to keep ozone in check. The risk of an ozone exceedance will remain Marginal due to the uncertainty in the precipitation forecast and air mass characteristics, with a focus on the western parts of the NMA and CMA.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 30, 2018
Valid: July 31-August 4 (Tuesday-Saturday)

Summary

The upper level synoptic pattern throughout most of the medium range period will be reminiscent of that from early last week, resulting in another multi-day stretch of strong moist southerly flow and relentless precipitation across the Mid-Atlantic, keeping a Slight risk of an ozone exceedance each day. Similar to last week, the location of the Mid-Atlantic between the Bermuda ridge of high pressure to the east and a stalling trough to the west will allow a tropical air mass to surge northward into the region promoting widespread heavy precipitation through Friday. Tuesday and Wednesday will be cloudy and soggy as a stalling trough over the Mississippi River Valley pulls a warm front northward across the NMA, and southerly flow feeds an atmospheric river of moisture flowing into the region. On Thursday, a weak cold front will push into the western Mid-Atlantic from the west before it seemingly stalls across the NMA and CMA due to weakening upper level support. The possibility for a few breaks in the clouds and light surface winds throughout the morning could be favorable for some ozone formation but continued southerly flow and widespread precipitation returning for the afternoon and evening should be enough to keep ozone in check. The primary forecast question for Thursday will be the timing and coverage of precipitation across the NMA and air mass characteristics behind the front given a shift to westerly flow. Although there is some uncertainty in the air quality forecast, the risk of an ozone exceedance will remain Slight. Friday will likely be the last day of widespread unsettled conditions as the stalled front across the NMA and CMA dissipates throughout the day. Another day of strong southerly flow will continue to feed the atmospheric river of moisture, promoting widespread precipitation across the region, with the heaviest likely occurring throughout the NMA and CMA. A pattern change looks like it will begin on Saturday. as the Bermuda High will build westward at mid-levels, bringing some relief from the relentless stretch of unsettled conditions. All-day precipitation along a surface trough draped along the I-95 Corridor will keep locations east of I-81 throughout the NMA and CMA clean. Mostly sunny skies, seasonable temperatures, and a shift to westerly flow will be favorable for ozone formation west of I-81. The primary forecast question for these locations will be how quickly the air mass is able to modify given the anticipated stretch of unsettled conditions through Friday. Given the likely clean air mass in place, the risk of an ozone exceedance will remain Slight on Saturday.

NWP Model Discussion

The weather models are generally in close agreement with the evolution of synoptic scale features until late in the medium range period. Similar to last week’s pattern, the Mid-Atlantic’s location between a stalled trough to the west and a strong Bermuda High to the east through Friday will promote strong southerly flow, bringing another stretch of unsettled conditions to the region. The NAM solution is the most aggressive of the three models through Wednesday, as it develops stronger shortwaves embedded in a longwave trough over the eastern U.S., resulting in a deeper trough than the ECMWF and GFS. Shortwave energy dropping southeastward across the Great Plains today will develop into a positively tilted longwave trough stretching across the Great Lakes, Mississippi River Valley, and into OK/AR by 12Z Tuesday. Similar to last week’s pattern, this trough will seemingly stall throughout Tuesday as it encounters a strong Bermuda High ridge in place over the Atlantic Ocean. As the upper level trough stalls over the Mississippi River Valley, embedded shortwave energy will round the base of the trough on Tuesday, lifting northeastward through the Ohio River Valley and into the Great Lakes on Wednesday, weakening the trough. The northward lifting of this shortwave energy will pick up the frontal boundary currently hung up across the southeastern U.S., developing a low pressure system that will pass through the Ohio River Valley on Wednesday. By 12Z Thursday, the shortwave energy that lifted northeastward will be absorbed into the northern stream flow as the weakening Mississippi River Valley through remains in place. On Friday, the axis of an upper level ridge over the Southwest U.S. will gradually tilt eastward, stretching northeastward into the Upper Midwest by 00Z Friday. At the same time, the Bermuda High ridge in the Atlantic will begin to build westward. The Mississippi River Valley through will begin to fall apart on Friday, in between the two ridges of high pressure. By about 18Z Saturday, broad ridging will set up across the most of the CONUS, including the Mid-Atlantic.

The Dailies

Day 1 (Tuesday): Tuesday will be soggy across the Mid-Atlantic as a warm front lifts northward across the NMA and southerly flow across the SMA feeds an atmospheric river of moisture into the Mid-Atlantic. Similar to last week, the location of the Mid-Atlantic sandwiched between a Bermuda ridge of high pressure to the east and a stalling trough to the west will allow a tropical air mass to surge northward into the region, promoting widespread and locally heavy precipitation. The WPC has the entire region under a Marginal risk of excessive rainfall with a Slight risk of excessive rainfall in the NMA and western SMA. Widespread precipitation, cloudy skies, sustained southerly surface winds, and strong southerly flow aloft will prevent any significant ozone formation from occurring on Tuesday. Despite the widespread coverage of clouds and precipitation, the air quality models keep the region under upper Good ozone with a few pockets of Moderate ozone throughout the eastern NMA/CMA, specifically along the I-95 corridor. The patches of Moderate ozone is likely in response to the warm front lifting northward in the NMA and associated converging surface winds, but they seem unlikely to verify. As a result of these conditions, the risk of an ozone exceedance will be Slight on Tuesday.

Day 2 (Wednesday): Widespread unsettled conditions will persist on Wednesday as southerly flow continues across the region ahead of a cold front slowly approaching the western Mid-Atlantic from the west. Another day of widespread showers and thunderstorms, cloudy skies, sustained southerly/southwesterly surface winds, and strong southerly flow aloft across the region will ensure Good air quality on Wednesday. The air quality models are in agreement on Wednesday but again appear to be too aggressive with ozone along frontal boundaries throughout the region. The air quality models develop Moderate ozone (upper Moderate range in BAMS-MAQSIP) across the western Mid-Atlantic where the cold front is forecast to be slowly pushing eastward. Given the flow pattern and widespread nature of unsettled conditions, ozone levels reaching the Moderate range seems unlikely in the Mid-Atlantic. This risk of an ozone exceedance will remain Slight on Wednesday.

Day 3 (Thursday): The cold front will finally push into the Mid-Atlantic on Thursday as it seemingly stalls across the NMA and CMA, given weak upper level support. Despite the arrival of the cold front, model guidance suggests that the majority of precipitation could hold off until the afternoon and evening hours. The possibility for a few breaks in the clouds and light surface winds throughout the morning could be favorable for some ozone formation but continued southerly flow and widespread precipitation throughout the afternoon and evening should be enough to keep ozone in check. The air quality models keep ozone levels elevated along and behind the cold front; specifically, the BAMS models develop Moderate/isolated USG ozone across the NMA with Good ozone throughout the rest of the region. Elevated ozone across the NMA is likely in response to less widespread precipitation, light surface winds, and a shift to westerly flow following the front, but USG ozone seems very unlikely. The primary forecast question for Thursday will be the timing and coverage of precipitation across the NMA and air mass characteristics behind the front given a shift to westerly flow. Although there is some uncertainty in the air quality forecast, the risk of an ozone exceedance will remain Slight.

Day 4-5 (Friday-Saturday): Friday will likely be the last day of widespread unsettled conditions as the stalled front across the NMA and CMA dissipates throughout the day. Another day of strong southerly flow will continue to feed an atmospheric river of moisture, promoting widespread precipitation across the region, with the heaviest likely occurring throughout the NMA and CMA. Unsettled conditions combined with strong southerly flow will keep ozone formation minimal on Friday. The BAMS air quality models are more reasonable on Friday as they keep just about the entire Mid-Atlantic under Good ozone with Moderate ozone creeping into northwestern PA. Given these conditions, the risk of an ozone exceedance will again remain Slight.

A pattern change looks like it will begin on Saturday. The Bermuda High building westward at mid-levels will bring some relief from the relentless stretch of unsettled conditions. Although scattered showers will remain along the east coast (along and east of I-81 through the NMA and CMA) and throughout the SMA, precipitation will not be as heavy, allowing most of the NMA and CMA to begin to dry out. Mostly sunny skies, seasonable temperatures, and a shift to westerly flow will be favorable for ozone formation west of I-81 throughout the NMA and CMA. The primary forecast question for these locations will be how quickly the air mass is able to modify given the anticipated stretch of unsettled conditions and upwind air mass characteristics. Throughout the rest of the region, southerly flow, scattered precipitation, and mostly cloudy skies will keep ozone formation in check. The risk of an ozone exceedance will remain Slight.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 27, 2018
Valid: July 28-August 1 (Saturday-Wednesday)

Summary

A Marginal risk of an ozone exceedance will persist through Tuesday as each day, there will be sufficient sunshine and light winds to potentially promote isolated rising ozone. Saturday will be pleasant throughout most of the Mid-Atlantic as weak high pressure begins to build in behind Friday’s cold front. Aside from a few lingering showers in the morning along the I-95 Corridor in the NMA and CMA and a few isolated afternoon showers across the NMA, the majority of precipitation will be confined along Friday’s cold front that will be hung up across the SMA. Mostly sunny skies, light westerly flow across the NMA and CMA, sea/bay breezes developing along the Atlantic coast and Chesapeake Bay, and the possible transport of remnant ON wildfire smoke transport from the west could be favorable for ozone formation across locations that remain dry. Upwind PM2.5 concentrations behind the cold front are quite low this morning, however, suggesting that the smoke (seen aloft in satellite imagery) is not mixing to the surface. The possibility for smoke transport and converging surface winds along the east coast will keep the risk of an ozone exceedance Marginal with a focus on locations along and east of the I-95 Corridor, especially areas affected by bay/sea breezes. The arrival of a drier air mass will bring comfortable conditions to most of the NMA and parts of the CMA on Sunday, while a developing low pressure system in the Midwest will promote unsettled conditions throughout the southern half of the region. The trend in yesterday’s and today’s weather models is to slow the northward progression of Friday’s lingering frontal boundary across the SMA as a warm front. As a result, it now appears the stalled front will remain across the SMA on Sunday. Light westerly flow and weak surface winds, in conjunction with the continued risk for smoke transport, could be favorable for some ozone formation in the NMA and CMA, but a mix of sun and clouds and lower Sunday emissions should be enough to prevent substantial ozone formation across the northern half of the region. Nevertheless, nearby surface high pressure, very light surface winds, and a mix of sun and clouds will keep the risk of an ozone exceedance Marginal on Sunday, with a focus on the I-95 Corridor, especially areas affected by bay/sea breezes. Unsettled conditions will impact the southern half of the region on Monday as a developing low pressure system over the Mississippi River Valley starts to slowly pull Friday’s cold front northward as a warm front, pushing into VA by Monday evening. The primary forecast question on Monday will be the coverage of clouds throughout the NMA and CMA as light westerly flow aloft, another day of weak surface winds, and near average temperatures could be favorable for ozone formation. One possible limiting factor along I-95 will be flow aloft turning weakly southerly, which may be enough to limit any rising ozone, especially along the southern and central portions of the Corridor (e.g., DC to PHL). Flow aloft will still be light westerly at locations farther inland, such as PIT, putting them and the northern branch of the Corridor at higher risk. The risk of an ozone exceedance will remain Marginal with a focus on locations along and just north of I-76/78 through PA and NJ and locations along the I-95 Corridor from BAL to NYC, given questions about the effects of weak southerly flow aloft and the northern extent of approaching precipitation. A low pressure system will move into the Ohio River Valley on Tuesday, promoting widespread showers and thunderstorms throughout the Mid-Atlantic. Widespread, locally heavy precipitation, cloudy skies, and sustained surface winds will limit ozone formation throughout the region. The primary forecast question on Tuesday will be how quickly unsettled conditions impact locations throughout the northeastern NMA. Given the uncertainty in the precipitation forecast and the recent trend for a slower northward migration of the warm front and associated precipitation, the risk of an ozone exceedance will remain Marginal with a focus on locations throughout the northeast NMA, especially along the I-95 Corridor. Precipitation will become more widespread on Wednesday as the Mid-Atlantic lies in the warm sector of the low pressure system centered in the Ohio River Valley. Strong southerly flow will continue to push a saturated air mass northward into the Mid-Atlantic promoting widespread precipitation and cloudy skies throughout the day, dropping the risk of an exceedance to Slight.

NWP Model Discussion

The weather models remain in close agreement with the evolution of synoptic scale features throughout the medium range period, although there are some slight differences that develop on Tuesday and Wednesday. By 12Z Saturday, an upper level low will be pushing eastward very slowly over the ON/QC border, reaching as far east as eastern QC by 00Z Sunday. Additional shortwave energy stretching across the northern half of the CONUS will keep broad upper level troughing across the CONUS into the work week. By 12Z Monday, another surge of shortwave energy will drop southward across the northern Great Plains, digging out a defined upper level longwave trough over the Central Plains by 12Z Tuesday. This feature will strengthen quickly on Tuesday, as the models briefly develop a closed low over the Mississippi River Valley by 00Z Wednesday. By 12Z Wednesday, the Mississippi River Valley trough will be influenced by a strong shortwave passing through southern Canada, which will pull the center of the Mississippi River Valley trough northward, resulting in a broad upper level trough centered over the Great Lakes region and Mississippi River Valley by 00Z Thursday. With an amplified longwave trough centered just to the east of the Mid-Atlantic and the western edge of the Bermuda High just off the east coast, the pattern through the first half of next week will be reminiscent of the pattern throughout this past weekend and the first half of this week, although somewhat weaker. Southerly flow will set up as early as 18Z Monday, gradually strengthening on Tuesday and Wednesday, promoting widespread unsettled conditions throughout the region early next week.

The Dailies

Day 1 (Saturday): Saturday will be pleasant throughout most of the Mid-Atlantic as weak high pressure begins to build in behind Friday’s cold front. Aside from a few lingering showers in the morning along the I-95 Corridor in the NMA and CMA, and a few isolated afternoon showers across the NMA, the majority of precipitation will be confined along Friday’s cold front that will be hung up across the SMA. Mostly sunny skies and light westerly flow across the NMA and CMA, in addition to possible remnant ON wildfire smoke transport from the west, could be favorable for ozone formation across locations that remain dry. Upwind PM2.5 concentrations behind the cold front are quite low this morning, however, suggesting that the smoke (seen aloft in satellite imagery) is not mixing to the surface. Weak westerly flow under mostly sunny skies across the NMA and CMA will allow for the development of sea/bay breezes that could enhance ozone formation along the Atlantic and Chesapeake Bay coasts. The air quality models are generally in agreement with widespread upper Good range ozone across most of the Mid-Atlantic, with a strip of Moderate ozone along the I-95 Corridor and around the Bay. This Moderate ozone is likely in response to weak westerly flow and the development of sea/bay breezes concentrating pollutants. The possibility for smoke transport and converging surface winds along the east coast will keep the risk of an ozone exceedance Marginal with a focus on locations along and east of the I-95 Corridor, especially areas affected by bay/sea breezes.

Day 2 (Sunday): The arrival of a drier air mass will bring comfortable conditions to most of the NMA and parts of the CMA on Sunday, while a developing low pressure system in the Midwest will promote unsettled conditions throughout the southern half of the region. Although the majority of the NMA and CMA will remain dry as surface high pressure moves over PA, a few afternoon showers cannot be ruled out as shortwave perturbations pass overhead. The trend in yesterday’s and today’s weather models is to slow the northward progression of Friday’s lingering frontal boundary across the SMA as a warm front. As a result, it now appears the stalled front will remain across the SMA on Sunday. Light westerly flow and weak surface winds, in conjunction with the continued risk for smoke transport, could be favorable for some ozone formation in NMA and CMA, but a mix of sun and clouds, along with lower Sunday emissions, should be enough to prevent substantial ozone formation across the northern half of the region. Throughout the SMA, Friday’s lingering cold front will promote cloudy skies and widespread showers and thunderstorms. These conditions, combined with southerly onshore flow, will keep ozone formation minimal across the SMA on Sunday. The BAMS air quality models are in agreement with scattered Moderate ozone east/south of I-95 in the NMA/CMA and a strip of Moderate ozone along the warm front across the SMA, with Good ozone elsewhere. The BAMS models possibly develop the Moderate ozone along the front in the SMA in response to converging surface winds but unsettled conditions will be widespread enough to keep ozone formation in check. Nearby surface high pressure, very light surface winds, and a mix of sun and clouds will keep the risk of an ozone exceedance Marginal, with a focus on the I-95 Corridor, especially areas affected by bay/sea breezes.

Day 3 (Monday): Unsettled conditions will impact the southern half of the region on Monday as a developing low pressure system over the Mississippi River Valley begins to slowly pull Friday’s cold front northward as a warm front, pushing into VA by Monday evening. Although a few scattered showers could develop in the NMA or CMA, the majority of precipitation will be confined to the SMA along and head of the lifting warm front. Southerly flow pushing a plume of moisture northward over the front will promote widespread showers and thunderstorms across the SMA. The primary forecast question on Monday will be the coverage of clouds throughout the NMA and CMA as another day of weak surface winds and near average temperatures could be favorable for ozone formation. One possible limiting factor along I-95 will be flow aloft turning weakly southerly, which may be enough to limit any rising ozone, especially along the southern and central portions of the Corridor (e.g., DC to PHL). Flow aloft will still be light westerly at locations farther inland, such as PIT, putting them and the northern branch of the Corridor at higher risk. The BAMS air quality models develop widespread Moderate ozone throughout the NMA and CMA with the highest ozone concentrated along the Mason-Dixon Line and stretching northeastward through NJ and into NY. Converging surface winds along the I-95 Corridor could aid ozone formation. The risk of an ozone exceedance will remain Marginal with a focus on locations along and just north of I-76/78 through PA and NJ and locations along the I-95 Corridor from BAL to NYC, given questions about the effects of weak southerly flow aloft and the northern extent of approaching precipitation.

Day 4-5 (Tuesday-Wednesday): A low pressure system will move into the Ohio River Valley on Tuesday, which will continue to pull the warm front northward, promoting widespread showers and thunderstorms throughout the Mid-Atlantic. The presence of this system to the west and the Bermuda High to the east will result in strong southerly flow that will push a moist air mass northward into the Mid-Atlantic. Widespread, locally heavy precipitation, cloudy skies, and sustained surface winds will limit ozone formation throughout the region. The primary forecast question on Tuesday will be how quickly unsettled conditions impact locations throughout the northeastern NMA. The air quality models suggest that precipitation could be slow to push into the northeastern NMA, as they develop Moderate and USG ozone throughout PA, NJ, and NY, north of the Mason-Dixon Line. Unsettled conditions throughout the rest of the Mid-Atlantic result in Good ozone in the air quality models. Given the uncertainty in the precipitation forecast and the recent trend for a slower northward migration of the warm front and associated precipitation, the risk of an ozone exceedance will remain Marginal with a focus on locations throughout the northeast NMA, especially along the I-95 Corridor.

Precipitation will become more widespread on Wednesday as the Mid-Atlantic lies in the warm sector of the low pressure system centered in the Ohio River Valley. Strong southerly flow will continue to push a saturated air mass northward into the Mid-Atlantic promoting locally heavy precipitation and cloudy skies throughout the day. Precipitation and cloud cover, combined with strong southerly flow sourcing from the Gulf of Mexico, will keep ozone formation minimal throughout the region on Wednesday, dropping the risk of an ozone exceedance back to Slight.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 26, 2018
Valid: July 27-31 (Friday-Tuesday)

Summary

Periods of sunny skies and a shift to westerly flow will warrant a Marginal risk of an ozone exceedance through most of the medium range period. Another weak cold front will push into the Mid-Atlantic on Friday, promoting pre-frontal convection moving west to east across the NMA and CMA. With flow aloft shifting westerly on Friday, there is a chance that remnant smoke from the ON fires may be advected into the region. This morning’s PM2.5 observations at upwind locations in the Ohio River Valley are generally in the Good range, however, suggesting that the threat from smoke has diminished, although the risk should continue to be monitored through the weekend. Coverage and timing of precipitation across the NMA and CMA will play a large role in Friday’s air quality forecast given conditions favorable for ozone formation will be in place ahead of the front, mainly along the I-95 Corridor across the eastern NMA and CMA. As a result, a Marginal risk of an ozone exceedance will remain in place, with a focus on the NMA/CMA I-95 Corridor and western NC. There remains uncertainty in the forecast for Saturday, but broad upper level troughing over the region will keep unsettled conditions possible despite surface high pressure building in behind Friday’s cold front. The primary forecast questions will be the coverage of afternoon precipitation/cloud cover across the NMA/CMA and regional air mass characteristics given the potential for smoke transport. Uncertainty in the precipitation forecast, the possibility for smoke transport, and converging surface winds along the east coast associated with bay/sea breezes will keep the risk of an ozone exceedance Marginal with a focus on locations along and east of the I-95 Corridor. A mix of conditions will impact the Mid-Atlantic on Sunday as the NMA and parts of the CMA will likely remain dry while unsettled conditions will blanket the SMA. The weather models have come into consensus today regarding the progression of Friday’s stalled frontal boundary, which will begin moving northward as a warm front late Sunday. Westerly flow aloft, mostly sunny skies, and near average temperatures could be favorable for ozone formation throughout the NMA and CMA on Sunday, but light northerly surface winds and lower Sunday emissions will keep the risk of an ozone exceedance Marginal with a focus on the I-95 Corridor. Unsettled conditions will impact most of the Mid-Atlantic on Monday as a developing low pressure system over the Mississippi River Valley continues to pull Friday’s cold front northward as a warm front. The northward push of precipitation will have a large impact on the air quality forecast throughout the NMA and CMA. As of now, it appears likely that precipitation will push through the CMA and as far north as I-76 in the NMA, allowing locations further north to stay dry. The risk of an ozone exceedance will remain Marginal with a focus on locations along and just north of I-76/78 through PA and NJ. Tuesday looks like a washout, with a synoptic pattern setting up that will be similar to the one from earlier this week. Strong southerly flow will push a saturated air mass over the warm front draped across the northern part of the NMA. A combination of widespread, locally heavy precipitation and strong southerly flow will drop the risk of an ozone exceedance to Slight.

NWP Model Discussion

The weather models come into general consensus today with the evolution of synoptic scale features throughout the medium range period. The upper level closed low and associated longwave trough that is currently moving very slowly across ON and the Great Lakes region will be centered over eastern ON by 12Z Friday when it essentially stalls, with broad upper level troughing covering much of the eastern CONUS. The flow pattern around this trough is concerning due to the presence of medium to heavy density wildfire smoke currently aloft over much of Canada and the northern tier of the U.S. (more about this in the Dailies). The center of the upper closed low will slowly pivot over the ON/QC border by 12Z Saturday as embedded shortwave perturbations drop across the Great Lakes, reinforcing a low amplitude trough axis stretching southward over the Ohio River Valley. This neutrally tilted trough axis will push eastward into the Mid-Atlantic by Saturday afternoon but will quickly be absorbed by a broader developing longwave trough over much of the northeastern CONUS. The broad longwave trough will arise as a result of additional shortwave energy dropping across the Great Plains and into the Midwest by 00Z Sunday. By 12Z Sunday, the ON/QC upper level disturbance will begin to pull northeastward well into QC but lingering shortwaves stretched across the Mid-Atlantic, Ohio River Valley, and Midwest will keep broad troughing throughout the northeastern U.S. By 00Z Monday, another surge of shortwave energy will be dropping across the Great Plains, reinforcing the broad upper level trough, which will develop into a defined and amplified trough axis over the center of the U.S. on Tuesday. A low pressure system associated with this stronger central U.S. trough will develop on Monday over the Mississippi River Valley and will push eastward towards the western Mid-Atlantic on Tuesday. This pattern will set the stage for an atmospheric river of moisture surging northward into the Mid-Atlantic on Monday and Tuesday, similar to the first part of this week, as the region will once again be sandwiched between the deep trough axis to the east and the persistent Bermuda ridge over the Atlantic Ocean.

The Dailies

Day 1 (Friday): Another weak cold front will push into the Mid-Atlantic on Friday, promoting unsettled conditions throughout parts of the NMA and CMA. The front is forecast to push into the western Mid-Atlantic from the northwest around 12Z Friday, reaching the I-95 Corridor around 00Z Saturday. It seems most likely that showers and thunderstorms associated with this front will hold off until the afternoon when the front is moving through the central NMA/CMA. This morning’s hi-res model guidance supports this outcome as the models develop scattered thunderstorms just east of the I-81 Corridor around 18Z, becoming more widespread through 00Z Saturday, at which time precipitation reaches the I-95 Corridor. SPC has the I-95 Corridor (from DC to NYC) under a Slight risk for severe thunderstorms, which suggests the storms that do form will likely be severe. Cold air advection aloft will promote below average temperatures behind the front, but periods of sunshine and westerly/southwesterly flow ahead of the front will favor slightly above average temperatures. Coverage and timing of precipitation across the NMA and CMA will play a large role in Friday’s air quality forecast given conditions favorable for ozone formation ahead of the front, mainly across the eastern NMA and CMA. With the shift to westerly flow aloft on Friday, there is a chance that remnant smoke from the ON fires may be advected into the region. This morning’s PM2.5 observations at upwind locations in the Ohio River Valley are generally in the Good range, however, suggesting that the threat from smoke has diminished. The majority of the SMA will remain dry but a humid air mass and mostly sunny skies will promote a few isolated showers and thunderstorms throughout the afternoon. Although mostly sunny skies and near average temperatures may be favorable for ozone formation, southerly flow should be enough to keep ozone in check across most of the SMA. The air quality models develop widespread Moderate ozone along and ahead of the front with widespread Good ozone behind the front. The large area of Moderate ozone is essentially along and east of I-81 through the NMA, CMA, and SMA with a few patches of USG ozone along the I-95 Corridor in the NMA and CMA and southwestern NC. Given ozone-friendly conditions in the SMA and ahead of the front in the NMA/CMA, the risk of an ozone exceedance will be Marginal with a focus on the I-95 Corridor and western NC.

Day 2 (Saturday): There remains uncertainty in the forecast for Saturday but broad upper level troughing over the region will keep unsettled conditions possible despite surface high pressure building in behind Friday’s cold front. Although the majority of precipitation will occur along what remains of Friday’s cold front throughout the SMA, shortwave perturbations moving over the northern half of the region will promote a few scattered showers across the NMA and CMA. In addition, the weather models suggest that precipitation will linger through Saturday morning and possibly into the early afternoon at locations along the eastern Mid-Atlantic, near the slow-moving cold front. Mostly sunny skies, near average temperatures and light westerly flow, in addition to possible remnant ON wildfire smoke transport from the west could be favorable for ozone formation across locations that remain dry. The primary forecast questions will be the coverage of afternoon precipitation/cloud cover across the NMA/CMA and regional air mass characteristics. Weak westerly flow under mostly sunny skies across the NMA and CMA will allow for the development of sea/bay breezes that could enhance ozone formation along the east coast. Although not completely in agreement, the air quality models have the same general outcome on Saturday with a mix of Moderate and upper Good range ozone throughout the Mid-Atlantic. The highest ozone is expected throughout the eastern NMA and CMA where the models develop widespread Moderate ozone with USG ozone along coastal regions, likely in response to sea/bay breezes developing. Uncertainty in the precipitation forecast, the possibility for smoke transport, and converging surface winds along the east coast will keep the risk of an ozone exceedance Marginal with a focus on locations along and east of the I-95 Corridor.

Day 3 (Sunday): A mix of conditions will impact the Mid-Atlantic on Sunday as the NMA and parts of the CMA will likely remain dry while unsettled conditions will blanket the SMA. The GFS has come around the ECMWF’s slower northward push of the stalled frontal boundary on Sunday, keeping the front across the SMA until late in the day, when it begins moving northward as a warm front. Nearby surface high pressure will push slightly less humid air into the NMA and CMA, promoting mostly sunny skies and light surface winds. Westerly flow aloft, mostly sunny skies, and near average temperatures could be favorable for ozone formation, but light northerly surface winds and lower Sunday emissions could be enough to keep ozone in check. In the SMA, southerly flow will push a moist air mass northward over the lingering frontal boundary draped across the SMA. Widespread showers and thunderstorms will keep ozone formation minimal throughout the SMA. The air quality models are not in agreement for Sunday as the NC-GFS2 and BAMS models diverge. The NC-GFS2 seems aggressive as it places most of the Mid-Atlantic under Moderate ozone, including the SMA where unsettled conditions are expected, and it develops a few isolated patches of USG ozone throughout the NMA and CMA. The BAMS models develop more of a mix of upper Good and Moderate ozone throughout most of the region with the highest ozone along and east of the I-95 Corridor in the NMA. Given these conditions, the risk of an ozone exceedance will remain Marginal with a focus on the I-95 Corridor from BAL to NYC.

Day 4-5 (Monday-Tuesday): Unsettled conditions will impact most of the Mid-Atlantic on Monday as a developing low pressure system over the Mississippi River Valley continues to pull Friday’s cold front northward as a warm front, reaching northern VA by 12Z Monday. Widespread showers and thunderstorms in addition to strong southerly flow will prevent ozone formation across the SMA. The northward push of precipitation will have a large impact on the air quality forecast throughout the NMA and CMA. As of now, it appears likely that precipitation will push through the CMA and as far north as I-76 in the NMA by Monday evening. Mostly sunny skies, light surface winds, and localized back trajectories ahead of the precipitation could promote ozone formation in locations across the NMA that stay dry. The air quality models are responding to these conditions as they develop a thin strip of Moderate ozone in the vicinity of I-76 with a few patches of USG ozone across eastern PA and NJ. The risk of an ozone exceedance will remain Marginal with a focus on locations along and just north of I-76/78 through PA and NJ.

The low pressure system to the west will push into the Ohio River Valley on Tuesday promoting widespread unsettled conditions throughout the entire Mid-Atlantic. Strong southerly flow will push a saturated air mass over the warm front draped across northern PA/central NJ. A combination of widespread, locally heavy precipitation and strong southerly flow will prevent significant ozone formation from occurring on Tuesday. The risk of an ozone exceedance will drop to Slight.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 25, 2018
Valid: July 26-30 (Thursday-Monday)

Summary

Unsettled conditions will keep a Slight risk of an ozone exceedance throughout the latter half of the medium range period but a lull in precipitation and cloud cover warrant a Marginal risk Thursday through Saturday.  Although most of the region will dry out on Thursday, Wednesday’s cold front will stall in in the vicinity of the I-95 Corridor, keeping unsettled conditions across the eastern portion of the region. While scattered showers and partly cloudy skies blanket the east coast, mostly sunny skies and light westerly surface winds could be favorable for ozone formation in the west.  In addition to these conditions, the latest satellite imagery reveals a large plume of moderate to high density smoke spread north/south across the Great Lakes region, originating from wildfires burning in ON.  This plume seems to be mixing to the ground across the affected region, with mid-Moderate PM2.5 observations in IN, KY and western TN this morning.  The possible transport of this remnant smoke may be an issue through the weekend, and should be monitored closely.  On Thursday, ozone-conducive weather in the west, in conjunction with possible smoke transport, will result in a Marginal risk of an ozone exceedance.  Another weak cold front will push into the region on Friday, keeping unsettled conditions likely throughout most of the region.  Despite the opportunity for scattered showers, light westerly flow, mostly sunny skies for most of the day, slightly above average temperatures, and possible continued smoke transport could be favorable for ozone formation along eastern portions of the CMA and NMA.  The risk of an ozone exceedance will remain Marginal on Friday.   Although surface high pressure will build eastward into the region following Friday’s weak cold front, broad upper level troughing overhead on Saturday will keep scattered unsettled conditions possible throughout parts of the Mid-Atlantic.  Mostly sunny skies, near average temperatures and light westerly flow, in addition to possible remnant ON wildfire smoke transport from the west could be favorable for ozone formation across locations that remain dry.  Given these conditions and uncertainty in the precipitation forecast, the risk of an ozone exceedance will remain Marginal with a focus on locations along the I-95 Corridor.  Model guidance continues to disagree on Sunday with the development of a low pressure system in the Midwest and associated warm front across the Mid-Atlantic.  Despite this uncertainty, surface winds will shift south/southeasterly throughout the afternoon, possibly keeping ozone in check.  The risk of an ozone exceedance will drop to Slight given these conditions in conjunction with lower Sunday emissions.  Although there is uncertainty in the progression of the warm front on Monday, unsettled conditions appear likely throughout the Mid-Atlantic.  Widespread precipitation and cloud cover will combine with strong southerly flow to keep ozone formation minimal.  The risk of an ozone exceedance will remain Slight on Monday.

NWP Model Discussion

The weather models are generally in consensus with synoptic scale features until the end of the medium range period where they continue to diverge with the strength of an upper level trough over the Midwest and associated warm front in the Mid-Atlantic.  Although the upper level trough currently over the eastern U.S. will be contracting to the northeast on Thursday, the arrival of a new longwave trough/closed low over southern ON will replace the departing eastern U.S. trough and keep upper level troughing over the Mid-Atlantic through the medium range period.  This new Canadian upper level trough/closed low will push eastward extremely slowly, only reaching the ON/QC border by 12Z Saturday.  As this upper level trough slowly progresses across southeastern Canada, additional shortwave perturbations will drop southward across the upper Great Plains, reinforcing broad upper level troughing across much of the northern half of the CONUS by 12Z Sunday.  The weather models continue to diverge with the strength and development of this reinforced longwave trough over the interior U.S. through the latter half of the weekend and into early next week.  The GFS develops stronger embedded shortwaves over the Midwest on Sunday, resulting in a more pronounced longwave trough, with its axis along the Midwest by 12Z Monday.  The ECMWF, in contrast, continues to be less aggressive with this feature, although it still develops a longwave trough in the same location as the GFS by 12Z Monday.  The differences in the strength of the trough translate primarily into the fate of Friday’s stalled front across the SMA.  The GFS brings this boundary northward as a warm front on Sunday, with associated widespread precipitation across the Mid-Atlantic.  The EC, continuing its trend from yesterday, holds the front across the SMA on Sunday, not bringing it northward until Monday, and thus keeping the NMA and CMA dry on Sunday.  The models seem to catch up with each other, in general, on Monday, with another round of strong southerly onshore flow setting up, as the new upper level trough abuts the persistent Bermuda ridge in the Atlantic.

The Dailies

Day 1 (Thursday): Although most of the region will dry out on Thursday, Wednesday’s cold front will stall in in the vicinity of the I-95 Corridor, keeping unsettled conditions across the eastern portion of the region.  While scattered showers and partly cloudy skies blanket the east coast, mostly sunny skies and light westerly surface winds could be favorable for ozone formation in the west.  In addition to these conditions, the latest satellite imagery reveals a large plume of moderate to high density smoke spread across the Great Lakes region, originating from wildfires burning throughout ON.  This plume seems to be mixing to the ground across the affected region, with mid-Moderate PM2.5 observations in IN, KY and western TN this morning.  The primary forecast questions for Thursday will be how far east the weak front (and associated precipitation) is able to push, the coverage of lingering cloud cover and precipitation across the western Mid-Atlantic, and the impacts of Canadian wildfire smoke transport.  The air quality models respond to these conditions by developing widespread Moderate ozone throughout much of the region, with a strip of upper Moderate/USG ozone around the Chesapeake Bay.  Although upper Moderate to USG ozone seems unlikely, given a presumably very clean air mass to start the day, along with morning southerly back trajectories and lack of continuous full afternoon sunshine, this feature in the air quality models is likely in response to the stalling front and associated converging surface winds.  So for now, we continue to discount this feature in the air quality models.  As a result, the risk of an ozone exceedance will be Marginal, with the best chance for Moderate ozone in the western part of the region.

Day 2 (Friday): Another weak cold front will push into the region from the west on Friday, keeping unsettled conditions likely throughout most of the region.  The front is forecast to enter the northwestern Mid-Atlantic in the morning hours and slowly push eastward, roughly reaching the I-81 Corridor by 00Z Saturday.  Precipitation appears likely throughout most of the western NMA and CMA with more scattered showers and thunderstorms across the eastern NMA and CMA.  While this newly arrived cold front promotes unsettled conditions throughout the NMA and CMA, what remains of Wednesday’s cold front will be draped across the eastern SMA, resulting in mostly cloudy skies and precipitation along the coast.  The air quality models continue to respond to the stalled frontal boundary along the I-95 Corridor as they develop a strip of USG ozone, while the rest of the region lies under Moderate and upper Good ozone.  Despite the opportunity for scattered showers, light westerly flow, mostly sunny skies and slightly above average temperatures, and continued smoke transport could be favorable for ozone formation.  The risk of an ozone exceedance will remain Marginal on Friday.

Day 3 (Saturday): Despite surface high pressure building eastward into the region following Friday’s weak cold front, broad upper level troughing overhead on Saturday will keep scattered unsettled conditions possible throughout parts of the Mid-Atlantic.  The majority of precipitation appears to be confined to the SMA as Friday’s cold front lingers in the eastern SMA, but a few showers are possible across the NMA and CMA as shortwave perturbations move overhead throughout the afternoon.  Mostly sunny skies, near average temperatures and light westerly flow, in addition to possible remnant ON wildfire smoke transport from the west could be favorable for ozone formation across locations that remain dry.  The primary forecast questions will be the coverage of afternoon precipitation/cloud cover across the NMA/CMA and regional air mass characteristics.  The air quality models keep the majority of the region under Good ozone despite continued westerly flow and mostly sunny skies.  The air quality models highlight locations along the east coast (south/east of I-95) with elevated ozone, likely in response to the development of sea/bay breezes throughout the afternoon and evening.  Given these conditions and uncertainty in the precipitation forecast, the risk of an ozone exceedance will remain Marginal with a focus on locations along the I-95 Corridor.

Day 4-5 (Sunday-Monday): Model guidance continues to disagree on Sunday with the development of a low pressure system in the Midwest and associated warm front across the Mid-Atlantic.  With its stronger upper level trough, the GFS develops a low pressure system in the Midwest by 12Z Sunday that will slowly pull the stalled frontal boundary in the SMA northward as a warm front.  The weaker ECMWF instead brings only a trough across the eastern U.S., allowing the stalled frontal boundary to remain in the SMA on Sunday.  If a solution like the GFS verifies, widespread showers and thunderstorms are likely along and ahead of the lifting warm front.  If a solution similar to the drier ECMWF verifies, conditions will likely remain dry throughout the NMA and CMA.  At this time, the WPC is splitting the difference and going with a consensus forecast regarding the warm front.  Regardless of the solution, surface winds will shift southerly throughout the afternoon, possibly keeping ozone in check.  The air quality models respond with Moderate ozone along the stalled frontal boundary throughout the SMA, and widespread Good ozone elsewhere.  The risk of an ozone exceedance will drop to Slight given these conditions in conjunction with lower Sunday emissions.

Although there is a great deal of uncertainty in the forecast for Monday, unsettled conditions appear likely throughout the Mid-Atlantic.  The models seem to catch up with the progression of the warm front, with resulting widespread precipitation and cloud cover that will combine with strong southerly flow to keep ozone formation minimal.  The risk of an ozone exceedance will remain Slight on Monday.

-Enlow/Huff